Power plant boilers typically are subject to an instant fuel flow interruption, or Main Fuel Trip (MFT), due to a multitude of safety and/or equipment protection reasons. When large boilers experience such an instantaneous fuel flow interruption, the hot gasses exiting the furnace rapidly contract as the furnace and flue gas temperatures decay. At the same time, the system Induced Draft and Booster fans continue to force flue gas through and out of the system, with the result that a vacuum condition can occur in the furnace, boiler casing and the associated ductwork upstream of the fan(s). Destructively high vacuum conditions resulting from these factors have caused boiler casing failures and ductwork collapses at numerous power plant installations.
Conventional methods of accommodating these occasional high vacuum conditions include the following:                1. Passive Protection. Passive protection is achieved by designing the boiler and ductwork to withstand the maximum negative pressure that could reasonably occur during a MFT or similar incident.        2. Active Protection. Active protection refers to implementing a mechanism or procedure for rapidly arresting or reducing the negative pressure-generating capability of the fan(s). This is typically accomplished by closing the fan inlet dampers, closing the fan inlet guide vanes, or changing the fan blade pitch for axial flow fans.        
While the passive protection method, namely, designing all components to withstand the maximum transient vacuum conditions, is effective, the expense is often excessive. For flue gas treatment system retrofits, re-design and stronger reinforcements for the boiler casing and ductwork may be prohibitively expensive.
The active protection method of control can often also be effective, but it always involves making one or more system compromises. This is because the repositioning of the large dampers or fan blades requires a significant period of time, and during this period of time negative pressures continue to build in the system. There are conditions at some plants where unacceptable negative pressure transients result in spite of these control actions. The generic problem is further exacerbated by the common use of large axial fans for many flue gas retrofit projects. These axial-type fans have inherently slower control/response actions, and hence tend to give larger transient vacuum conditions for a MFT incident.
Accordingly, the prior art approaches to this problem are expensive, are only effective some of the time, and/or impose serious system design and operation limitations.